THE GENETICS OF SPONTANEOUS TUMOR FORMATION 263 



Table 11 



The close similarity of the lung tumor incidence in the high tumor stock 

 and in the Fi hybrids is striking. 



There is evidently a small percentage of potentially and genetically lung 

 tumor animals which fail to develop the neoplasms to a degree or at a 

 sufficiently early age to be recorded. This percentage may be taken as 11.3 

 which represents an average of lo.o and 12.5, the A stock and Fi percentage 

 respectively of normal overlaps. 



Using 88.7% as the incidence of lung tumors in a stock in which all 

 animals carried the hypothetical dominant gene for these neoplasms, we may 

 calculate the expectation for F2 as 75% of that figure or 66.5. The actual 

 percentage observed in that generation was 67.3. The close degree of corre- 

 spondence between the calculated and observed figures is strong evidence in 

 support of the theory that a dominant Mendelian gene may, in certain 

 crosses, play the main role in determining the incidence of these tumors. 



The situation is not, however, quite so simple. Certain crosses of other 

 stocks recently made by Heston (unfinished data) and reported at the 1940 

 meeting of the American Association for Cancer Research show that modify- 

 ing genes or other genetic agents influence the percentage of lung tumors 

 formed. Not all ''low" tumor stocks behave in a similar manner when 

 crossed with a single "high" tumor strain. 



Furthermore, the percentage of mice showing multiple nodules in the 

 lungs was quite different when certain "low" tumor strains were employed 

 from what it was when others were used. 



The age at which the nodules became visible also varied according to the 

 parent strains used. 



We can thus conclude that the available evidence suggests that a domi- 

 nant gene is at times clearly indicated but that its influence is subject to 

 modification by secondary genes which affect actual incidence of any 

 lung nodule, the number of nodules, and the age at which they are usually 

 formed. 



